This invention relates to a ceramic product, and then particularly one for handling molten aluminium. In this specification the term xe2x80x98ceramic productxe2x80x99 refers to a product which has been formed by firing a ceramic raw material to a suitable temperature.
Molten aluminium, which has a relatively low melting point of approximately 700xc2x0 C., is difficult to handle and for this reason the apparatus used usually comprises a product that has a high thermal shock resistance, a high resistance to weftability, a low thermal conductivity/capacity, and high strength.
None of the products which has hitherto been used for the aforesaid purpose, amongst others aluminium titanate; fused silica; cast iron and fibrous ceramic board, has ideal properties in this regard.
It is an object of this invention to provide a ceramic product which the applicant believes has advantages over the known arrangements.
According to the invention a ceramic product is provided which contains at least 70% lithium aluminium silicate on a mass per mass basis.
Lithium aluminium silicate (hereafter referred to as LAS) occurs naturally as the mineral petalite, and is used in small quantities in ceramic formulations instead of feldspar in order to decrease the thermal expansion of such formulations, and hence improve their resistance to thermal shock. It is also used in glass compositions for stove tops and oven ware.
Applicant has now found that an LAS-ceramic product according to the invention has extremely low thermal expansion and can hence as such be used in any apparatus subjected to any form of heat exposure, such as for example, that occurring in the handling of molten aluminium.
Such an LAS-ceramic product can be manufactured by forming a suitable quantity of finely divided petalite in any of the conventional methods such as slip casting; extrusion or injection moulding; and wet or dry pressing and then firing it at a temperature in the order of between 1150xc2x0 C. and 1210xc2x0 C.
For use in the aluminium industry, such an LAS-ceramic product does not have to be fired to full density. It has been found that fired densities of the order of 60 to 70% of the theoretical are obtained when firing at between 1150 and 1210xc2x0 C. At these densities, the product shows no dimensional changes, no thermal expansion, adequate strength, a low thermal conductivity and thermal capacity, and also takes an excellent coating when painted with protective materials such as boron nitride, as is normal practice in the aluminium industry. It can furthermore be impregnated with liquid substances intended for enhancing the metallurgical properties of the product.
If required, the density of the LAS-ceramic product can be lowered by the addition of fly ash microspheres and/or a suitable combustible material.
Further according to the invention the LAS-ceramic product according to the invention includes a non-wetting agent which has been incorporated in the fired LAS-ceramic product.
The reason for such incorporation is that the inherent wettability of LAS by molten aluminium poses severe problems when the protective coating usually provided on the LAS-ceramic product is removed, which may, for example, happen during use of the ceramic product.
It has furthermore been found that such wettability cannot be reduced by the incorporation of known non-wetting agents in the ceramic formulation of the product according to the invention, because such agents tend to react with the LAS during the firing process causing them to lose their ability as non-wetting agents.
Furthermore, by incorporating the non-wetting agent in the fired LAS-ceramic product, the thermal expansion properties of the ceramic product are not adversely affected.
Further according to the invention the non-wetting agent comprises barium sulphate (BaSO4), which is a well known non-wetting agent in the ceramic industry.
Further according to the invention a method for improving the non-wettability of an LAS-ceramic product is provided which includes the step of providing a suitable non-wetting agent such as BaSO4 in the fired LAS-ceramic product.
In one form of the invention the BaSO4 may be provided in the fired LAS-ceramic product by impregnating the semi-porous fired LAS-ceramic product with a saturated aqueous solution of barium sulphide (BaS) which, after drying, is oxidised in situ to BaSO4.
Such oxidation may be effected by calcining the dried product to 500-800xc2x0 C. in an oxidising atmosphere.
Commercial BaS can be used, or BaS can be prepared by the reduction of BaSO4 with carbon at 1190xc2x0 C., or with ammonia at 1000xc2x0 C.
In another form of the invention the BaSO4 may be provided by impregnating the semi-porous fired LAS-ceramic product with a solution of BaCl2, and then treating the product with sulphuric acid to cause BaSO4 to precipitate inside the pores of the ceramic product.
Preferably the BaCl2-solution is a substantially saturated one, and the sulphuric acid comprises diluted sulphuric acid.
The BaCl2xe2x80x94solution can either be prepared by dissolving BaCl2 in water, or by treating BaCO3 with hydrochloric acid.
In a preferred form of the invention the ceramic product according to the invention comprises substantially 100% LAS (mass per mass).